1. Field of the Invention
The present invention relates generally to circuit design, and more particularly, to providing security for data stored on integrated circuits such as application specific integrated circuits (ASICs).
2. State of the Art
It is well known to design circuits, such as ASICs, with internal security features to inhibit access to stored data. For example, ASICs have been designed with data encryption chips for encoding or decoding data stored in the chip. A data encryption key is included on the chip, and the key must be used to properly decode the data stored on the chip. Electrical measures for further enhancing internal security of stored data include eliminating external access to memory address circuitry of the chip to render it difficult to read out code.
In some cases, a hierarchy of both physical security measures and internal security measures are used to provide plural layers of protection and to ensure the highest security of stored data. For example, physical security measures include forming one or more protective layers over a memory device, such as a read only memory (ROM), a programmable read only memory (PROM) or an erasable PROM (i.e., EPROM), to limit visual access to the memory even if the material used to encapsulate the chip is removed. Other known security techniques include using polysilicon layers to carry signals (thereby rendering signal transmission invisible). Criss-crossing of signal paths on a multi-layer chip can also be used to wind conductive paths through multiple layers such that removal of an upper layer eliminates any possibility of probing signal paths located in lower layers.
While the foregoing techniques, in addition to other well known techniques, provide a relatively high level of both internal and physical security for data stored on a chip, the widespread pirating in the market place makes it necessary to continue to develop more stringent levels of security.